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Objective: The association of American Medical Association's (AMA) Guides edition with impairment ratings is uncertain.

Methods: We used data from a consecutive sample of 249 injured workers referred for an independent evaluation 10 months before and after assessors switched from the 5th to the 6th edition of the AMA Guides.

Results: The median whole person impairment rating was 7.0% (interquartile range [IQR]: 4 to 14) for 131 claimants assessed with the 5th edition of the Guides, and 4.0% (IQR: 2 to 8) for 118 claimants assessed with the 6th edition (P-value for difference: 0.002). Multivariable analysis showed a 36.4% relative reduction (95% confidence interval [CI] 17.2% to 57.3%) in impairment rating with the 6th edition of the Guides versus the 5th edition.

Conclusions: The 6th edition of the AMA Guides provides systematically lower impairment ratings for injured workers than the 5th edition.

Contributors: R.W.P. and J.W.B. conceived the study design; M.M.dV., S.J.H., and L.W.A.H.vB. acquired the data; B.S. and S.M.K. performed the data analysis; J.W.B., M.M.dV., S.J.H., L.W.A.H.vB., and R.W.P. interpreted the data and findings; J.W.B., B.S., and R.J.C. drafted the manuscript; R.W.P. and J.W.B. provided the methodological support and study supervision. All authors reviewed the study findings and read and approved the final version before submission. R.W.P., J.W.B., and B.S. have full access to all the data in the study and the final responsibility for the decision to submit for publication.

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